1.5+ savegames are stored in seperate places from 1.42-, and apparently it doesn't move existing saves to the new location during the patch installation. The 1.5+ version stores things in the saved games folder on vista/w7, and I think local directory on XP? Where as 1.42- stored them in the local directory. I think if you are using vista or W7, and put 1.5.11 or whatever in XP compatability mode, then that may make it look in the local directory and find your old saves from the previous version. The saves are all perfectly compatible.

Obviously though this is of little concern to you, since 1.5+ didn't really work out, but the above is the state of affairs, anyway...